Titanium dioxides are useful materials due to their interesting properties and important applications. A common use for Titanium dioxide (TiO2) is as a photocatalyst. In order to be used as a photocatalyst, titanium dioxide has to exhibit crystallinity. Among the crystalline phases of titanium dioxide, anatase is reported to show the highest performance. The preparation of anatase thin films is usually achieved at temperatures above 400.degrees celsius. However, the need for high temperature processing limits the choice of substrates to heat resistant materials. The growth of anatase titania nanocrystallites at low temperatures is important for the fabrication of nanocrystalline titania films on substrates with low heat resistance, such as wood, paper, plastics, textiles and biomaterials.
Thus, in a case where amorphous titanium dioxide is formed from a titanium-based starting material through hydrolysis and condensation polymerization, it is a necessity to carry out a heat treatment at high temperature, i.e., sintering process, in order to convert the amorphous titanium dioxide to an anatase type titanium dioxide. It is known that the sintering temperature is typically around or above 400.degrees celsius.
Accordingly, it is fundamentally impossible to form anatase titania or a composite oxide containing anatase titania, for example, on substrates having low resistance to heat such as organic polymer substrates or living tissue having low resistance to high temperature, low pressure and high pressure.
It has been reported that anatase nanocrystallites can be grown on cotton fabrics using a boiling water treatment (W. A. Daoud and J. H. Xin, J. Am. Ceram Soc., 87 [5] 953-55 2004). Even though, the processing temperature could be reduced to 100 degrees celsius, the need for a post-preparation heat treatment is still there. However, the in-situ preparation of anatase oxides at temperatures of low temperatures is of great importance not only from the point of view of energy saving, as annealing post-treatment is avoided, but also it would open up new applications in new fields such as biochemistry.
Among conventional methods using titanium dioxide as a photocatalyst, some can be distinguished as the most widely used methods: a first method of using an anatase type titanium dioxide in the powder form and a second one of using a thin film of the anatase type titanium dioxide formed on a specific support. Although the former is superior to the latter in the photocatalyst activity due to the relatively large surface area of the titanium dioxide prepared, the latter is more practicable in the aspect of stability of the titanium dioxide. Therefore, the actually used photocatalysts are prepared by the second method that involves formation of a titanium dioxide film on a support by means of a sol-gel method.
The process used in conventional methods to prepare anatase materials includes the steps of: (a) preparing a titanium oxide precipitate from an aqueous solution of a titanium-based starting material by hydrolysis and condensation polymerization; (b) subjecting the precipitate to filtration to obtain a white amorphous titanium dioxide; (c) sintering the amorphous titanium dioxide at high temperature (above 400 degrees celsius.) to obtain an anatase type titanium dioxide; (d) milling the resulting titanium dioxide to yield a powdery titanium dioxide; (e) dispersing the powder in a specific solvent to prepare an anatase type titanium dioxide solution; and (f) coating the solution on a support.
Such a conventional method for preparing a titanium dioxide photocatalyst is a multistage process that involves the steps of filtration, sintering, milling and dispersion, which may cause an increase in the unit production cost. The conventional method is also a troublesome in that there is a need of milling the anatase type titanium dioxide into very tiny particles, i.e., in the particle size of nano level, and dispersing the particles in a specific solvent prior to a coating step, in order to enhance the outer appearance of the finally coated photocatalyst or to increase the surface area of the photocatalyst. With a large particle size of the anatase type titanium dioxide, an excessively large amount of the precipitate may be formed in the dispersion step and the resulting photocatalyst is inapplicable to a coating solution.
Also, the use of several additives for enhancing the coating characteristics and the hardness of the photocatalyst films may cause a problem such as deteriorating the stability of the titanium dioxide disperse solution and resulting in formation of precipitates.
The most well-known anatase type titanium dioxide powder commercially available is P25 supplied by Degussa, which is now manufactured in a limited number of countries. There are also commercially available coating products containing a photocatalyst dispersed in water and ethanol, which are too expensive due to their expensive preparation process.